This disclosure relates to self-assembled structures, methods of manufacture thereof and to articles comprising the same.
Block copolymers form self-assembled nanostructures in order to reduce the free energy of the system. Nanostructures are those having average largest widths or thicknesses of less than 100 nanometers (nm). This self-assembly produces periodic structures as a result of the reduction in free energy. The periodic structures can be in the form of domains, lamellae or cylinders. Because of these structures, thin films of block copolymers provide spatial chemical contrast at the nanometer-scale and, therefore, they have been used as an alternative low-cost nano-patterning material for generating nanoscale structures. While these block copolymer films can provide contrast at the nanometer scale, it is often difficult to produce copolymer films that can display periodicity at less than 60 nm. Modern electronic devices however often utilize structures that have a periodicity of less than 60 nm and it is therefore desirable to produce copolymers that can easily display structures that have average largest widths or thicknesses of less than 60 nm, while at the same time displaying a periodicity of less than 60 nm.
Many attempts have been made to develop copolymers that have minimum pattern dimensions of less than 60 nm, while at the same time displaying a periodicity of less than 60 nm. The assembly of polymer chains into a regular array, and especially a periodic array, is sometimes referred to as “bottom up lithography”. The processes for forming periodic structures for electronic devices from block copolymers, which are aligned to guiding features on the substrate within lithography are known as “directed self-assembly’. However, four of the challenges in trying to build a workable electronic device from a periodic array have to do with a) the desire to register or align that periodic array with great precision and accuracy to the underlying elements of the circuit pattern, b) secondly the desire to form non-periodic shapes in the pattern as part of the electronic circuit design, c) thirdly the ability for the pattern to form sharp bends and corners and line ends as part of the circuit design pattern layout design, and d) the ability for the pattern to be formed in a multitude of periodicities. These limitations with bottom-up lithography using periodic patterns formed from block copolymers have resulted in the need to design complex chemoepitaxy or graphoepitaxy process schemes for alignment, pattern formation and defect reduction.
Conventional ‘top down’ lithography, which creates patterns through projection and focusing of light or energetic particles through a mask onto a thin photoresist layer on a substrate, or in the case of electron beam lithography may involve projection of electrons through an electromagnetic field in a pattern-wise manner onto a thin photoresist layer on a substrate, has the advantage of being more amenable to conventional methods of alignment of the pattern formation to the underlaying elements of the circuit pattern, and being able to form non-periodic shapes in the pattern as part of the circuit design, being able to directly form line ends and sharp bends, and the ability to form patterns in a multiplicity of periodicities. However, top down lithography, in the case of optical lithography, is constrained in the smallest pattern it can form, as a result of the diffraction of light through mask openings whose dimension is similar or smaller than the wavelength, which causes loss of light intensity modulation between the masked and unmasked regions. Other important factors which limit resolution are light flare, reflection issues from various film interfaces, imperfections in the optical quality of the lens elements, focal depth variations and tilt, vibrations during exposure, photon and photoacid shot noise and line edge roughness. In the case of electron beam lithography, the smallest useful pattern sizes which can be formed are limited by the beam spot size, the ability to stitch or merge writing patterns effectively and accurately, electron scatter and backscatter in the photoresist and underlying substrates, electron and photoacid shot noise and line edge roughness. Electron beam lithography is also highly limited by throughput, since the images are patternwise formed pixel-by-pixel, because as smaller pixel dimensions are required for smaller pattern sizes, the number of imaging pixels per unit area increases as the square of the pixel unit dimension.